qmlsc/qmlcachegen should be able to handle QList<registered object type>

Currently we automatically register QList<T> if T is a value type, and QQmlListProperty<T> if T is an object type.

However, the engine also happily handles QList of object types. qmllint and qmlsc/qmlcachegen should arguably not warn about them (which they do for the attached project, though qmllint only does so if warnings are enabled).

A workaround is to do

struct Helper
{
    Q_GADGET
    QML_FOREIGN(QList<DataObject *>)
    QML_ANONYMOUS
    QML_SEQUENTIAL_CONTAINER(DataObject *)
};

but that should most likely not be necessary.